The invention is related to circuitry that is associated with photosensitive elements. While the circuitry is useful generally in light sensing and measuring, it is mainly of interest in photographic applications.
In the area of photography, automatic cameras employ light sensors that regulate the shutter and/or lens opening to control film exposure. Such a system should be capable of operating under full sunlight, or about 1500 foot candles of sky illumination, to a dimly lit scene at about 0.02 foot candle or even less. Thus the photosensor should function over about five orders of magnitude and desirably the response should be linear. Since flash exposure is often used on cameras, the photo sensor should have a response time that is fast enough to react to an electronic flash as well as the more common flash bulb. The former device typically emits light pulses having a duration in the millisecond or less region.
Photoelectric light sensors have been available for many years and the practice has developed of using a particular device for a particular light level and/or application. Photometry in particular has advanced with the evolution of many different types of devices. For example, when very low light levels are to be sensed, cadmium sulfide cells are often used. While they are very sensitive, they are nonlinear and slow to respond to changes in light and in addition possess an undesirable "memory" of previous high light level. A sensitive cadmium sulfide cell might respond to an electronic flash, but its response would be greatly reduced as a function of the flash speed and the elapsed time from the preceding flash. Selenium has long been used because it is self-generating and moderately fast, but it will not operate well at very low light levels and such cells are typically very large. Silicon is an excellent photoresponsive material. Its response to changes in illumination is very rapid and linear. It will respond to very intense light levels and, while it will respond linearly down to very low light levels, the photo current at low level is so small as to be very difficult to be sensed or measured. Therefore, while silicon is probably one of the best overall photosensitive materials and is quite amenable to device fabrication, its use has been curtailed substantially because of a lack of suitable associated circuitry.